Abstract

The microstructure development was investigated in torsion deformed NiAl. High strain torsion of solid bars was done with a Paterson rock deformation machine at temperatures between 700 K and 1300 K under a confining pressure of 400 MPa. The maximum shear strains and shear strain rates applied were 19 and 2.2×10−4s−1, respectively. The shear stress–shear strain curves are characterized by a peak at low shear strains, which is followed by softening and a steady state at high shear strains. Increasing shear strain leads to grain refinement, with the average grain size decreasing with temperature. Moreover, a steady state grain aspect ratio and inclination of the grain long axis with respect to the shear plane is observed. With increasing shear strain, the fraction of low angle grain boundaries goes over a maximum and approaches a steady state of about 20–40%. The development of the microstructure is characterized by two different temperature regimes. Up to 1000 K, continuous dynamic recrystallization characterized by limited grain growth takes place, leading to a transformation of low into high angle grain boundaries. At temperatures above 1000 K, discontinuous dynamic recrystallization occurs by massive grain growth. The results are qualitatively discussed on the basis of models dealing with dynamic recrystallization.

Figures

(a) Optical micrograph of a torsion deformed NiAl sample. The sample has been enclosed in a steel jacket, the strain markers of which indicate shear deformation. (b) Schematic of a torsion sample of length l, which has been twisted by the angle Θ. A shear plane (gray area) and shear sense (arrows) are indicated. r denotes the radial distance from the center of the bar.

Grain size subject to shear strain. Samples of different initial orientations are compared at the same deformation temperatures. Open symbols correspond to the respective higher strain rate (the initial grain size is about 50 μm).

Fraction of LAGBs subject to shear strain. Samples of different initial orientations are compared at the same deformation temperatures. Open symbols correspond to the respective higher strain rate. The initial value is indicated by a star.

Grain alignment in terms of the inclination angle Φ′ subject to shear strain (arithmetic mean). Samples of different initial orientations are compared at the same deformation temperature. The solid curve corresponds to Eq. 2. Initial ⟨100⟩: plots at 800 K and 900 K correspond to samples 100–8002 and 100–9002, respectively.

Mean recrystallized grain size as a function of steady state axial stress (equivalent stress). The transformation of shear stress τ into equivalent stress σ has been made according to σ=3(1+n)/2nτ(n=stress exponent) (10). The straight line refers to the relation d=K/σ, with K=800 μm MPa. Compression data are from Ref. 15.

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